The elimination of human schistosomiasis as a major public health problem will require an integrated approach involving several different control measures. The development of an effective vaccine may constitute an important part of this overall approach. Among the models available for the study of potential vaccines, infection of inbred mice by highly irradiated Schistosoma monsoni cercariae offers reproducible and significant protective immunity to a subsequent, unattenuated homologous challenge. It is the primary purpose of this proposal to: Examine the developing immune reactivity in mice immunized by exposure to highly irradiated cercariae. A parallel examination will be performed of the immune reactivity in (a) mice harboring a patent S. mansoni infection, and (b) mice immunized with nonviable schistosome preparations. In vitro analysis of lymphocyte reactivity will include lymphocyte blastogenesis and lymphokine production, and humoral profiles will be determined by a series of standard serological methods. Examine pathogenesis of cercarial penetration of the skin in the above model, and assess histologically the reactivity to selected cercarial and schistosomular preparations. Examine cellular and humoral components in contribution to protective immunity. Analyze and compare inbred mouse strain dependency for development of protective immunity following irradiated cercarial exposure.